The BBC Trust has today given the green light to the corporation's iPlayer on-demand TV service, but with significant changes including some proposed by Ofcom.

Changes proposed by the trust as part of its public value test for the iPlayer include limiting the storage window for seven-day catch up of individual TV shows to 30 days, rather than the 13 weeks requested by BBC management.

The trust has also taken on board Ofcom's reservations about the market impact of the iPlayer's series stacking facility.

Series stacking will only be available for certain kinds of programme, the trust has ruled, "with a distinct run, with a beginning and end, and a narrative arc or those which are landmark series with exceptionally high impact".

EastEnders, Horizon, Top Gear and Blue Peter would be excluded from series stacking under the trust's iPlayer proposals.

But series such as Bleak House, Planet Earth, Doctor Who and Strictly Come Dancing would be included.

The trust has also asked BBC management to ensure that the iPlayer on-demand TV services work on all computer operating systems, not just with Microsoft applications.

The original iPlayer proposal would have made the service available only to computer users with Windows XP and Windows Media Player 10.

The trust will also draw up a policy for syndicating BBC on-demand content to other internet operators such as Google.

The trust's proposals will now be open for further consultation, with a March 28 deadline for submissions. The Trust is aiming to give final approval by May 2.

As part of its on-demand submission, BBC management had requested to be allowed to offer audio books and classical music performances as podcast-style audio downloads.

But the trust ruled that the potential market impact of such downloads outweighed their public value, and said it would not permit them.

The trust has also asked BBC management to draw up options for how it will provide parental controls to protect children from viewing unsuitable content using its new iPlayer download service.

Other issues arising from the public value and market impact assessments included the question of whether the iPlayer should feature content from sources other than the BBC.

BBC management did not specifically ask the trust to consider the issue of third-party content, but it was raised by some contributors to the public value test consultation.

The trust said it had not reached a conclusion on the public value of offering third-party content via the iPlayer - but added that it was not giving permission for such a feature to be added at this stage.

A feature which had appeared in some demos/promotional material enabling iPlayer users to bookmark a show for download ahead of transmission was not included in the trust's approval.

The trust said the various iPlayer on-demand services - offering a seven-day catch up via internet and cable, series stacking catch up, simulcast streaming of BBC TV channels and podcasts - would cost an estimated £20m-£30m a year, or £131m over the next five years.

"Our view is that the BBC's new on-demand services are likely to deliver significant public value, and should be allowed to proceed, but subject to certain conditions in order to reduce the potential negative market impact," said Diane Coyle, the BBC Trust member who chairs the public value test steering group.

BBC management said in a statement that it would consider the trust's decision to limit the scope of the iPlayer service and respond in due course.

The iPlayer's series stacking facility, which allows viewers to catch up with episodes of a series that they may have missed, will only apply to new shows, not repeats.

In its public value test conclusions, the trust said it was limiting the storage of seven-day catch up downloads on computers to 30 days because the 13-week window requested by BBC management might hurt the corporation's ability to exploit secondary rights to its shows, including from DVD sales.

The trust also noted Ofcom's view that a 13-week catch up storage limit could hit the commercial TV on-demand market, and that a 30-day viewing window had been adopted by several other broadcasters for their download services.

Christopher Woolard, the BBC Trust governance unit's head of compliance and value for money, said: "It's a delicate balance the trust needs to strike. Some people would say 'I'd like everything free forever, please'. But secondary rights can be exploited commercially and then that money goes back into BBC programmes."

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